Hard to say without you being more specific...
When you say 6 cupboards do you mean 6 single robes, 3 double robes (6 doors) or 6 double robes?? Also drawer stacks included in the wardrobes (combi style?) or separate? If separate are you wanting single width, double width 3 drawer 5 drawer 4+2 drawer? Any other internals like trouser rails, tie racks, internal drawers etc?
Then when you say fully fitted do you mean wall to wall or do you mean against one wall with end panels scribed to the walls/skirting's? What height wardrobes are you looking at? All the way to the ceiling or would you prefer standard 2.1m with cornice to finish off the top?
What type of doors do you want? Slab style, shaker style, fully wrapped, partial wrap, mirror? Gloss or matt finish and the all important colour?!
Prices will vary wildy depending on your requirements and the place you go and I can only comment on the highstreet retailers as I have no experience with online companies.
The specialists like Sharpes will not even entertain a project under £1500 with the average cost somewhere around £4k although I have seen as high as £10k. The big sheds (B&Q, Homebase, Wren) will supply you a less bespoke product, as in they use standard unit sizes normally 500mm wide by 600mm deep, which comes flatpacked but are usually significantly cheaper. I tend to find that if you have any combed ceilings that the product supplied by the likes of B&Q and Homebase will not be suitable for your needs and you will need to look at a more bespoke specialist product.
Homebase supply Schreiber fitted wardrobes which are well built with excellent quality internals although lacking in some of the colour and style choices of the other places.
B&Q offer their own brand of fitted wardrobes which look nice and have a decent range of styles and colours but I dont rate the quality very much and their internal fittings are a bit cheap and nasty.
I have heard nothing but bad things about Wrens own brand fitted bedrooms from fitters and customers alike. Damaged deliveries, units that dont fit, very poor quality finish on the door wraps and end panels etc.
Cant comment on Ikea as I have no experience with them but I would expect them to fall somewhere between the Homebase and B&Q product with lots of options for style and internal fittings.
Price wise I can only speak for Homebase but I priced 3 double robes 2x 3drawer chests plus fitting today for around £1400 although this was a fairly simple affair. Typically I would think you are looking at around £2k for a fully fitted solution.
/Salsa